Carbapenems are the most potent β-lactam antibiotics. In many instances, these lifesaving drugs are considered “last line agents” due to their activity against many antibiotic resistant Gram-negative bacteria. Unfortunately, carbapenem-resistant bacteria are rapidly emerging as a cause of opportunistic healthcare-associated infections, particularly in immunocompromised individuals. As a result, antimicrobial treatment of these carbapenem-resistant pathogens is becoming limited. Widespread dissemination of carbapenem-resistant Gram-negative bacteria presents a profound challenge to effective healthcare.
Production of hydrolytic enzymes (β-lactamases) is a key mechanism of β-lactam resistance in Gram-negative bacteria. For a long time, carbapenem-hydrolyzing β-lactamases (carbapenemases) were infrequent. There are two types of β-lactamases with carbapenemase activity: those that use a serine residue as the nucleophile to inactivate the antibiotic (i.e., serine carbapenemases), and those that use Zn2+ ions to activate a nucleophilic water molecule (i.e., metallo-β-lactamases, MBLs).
MBLs are the most potent carbapenemases. MBLs are broad spectrum enzymes, being able to hydrolyze equally well penicillins and cephalosporins (FIG. 1). In the 1980s to 1990s, only a few MBLs were known, and these were limited to strains of restricted clinical impact. However, MBLs are now increasingly encountered in important Gram-negative pathogens, including Enterobacteriaceae and non-fermenting species. Multiple MBLs have been found on mobile genetic elements and have consequently disseminated worldwide among pathogenic and opportunistic bacteria. For instance, variants of the NOM, IMP and VIM MBLs are now globally distributed, SPM-1 is present in Pseudomonas aeruginosa across South America, and has recently been identified in Europe.
In particular, NDM-1, is raising significant concern because: i) it is located in a mobile genetic background with multiple resistance elements; ii) it is becoming prevalent in Enterobacteriaceae; and iii) it has now been identified in food and water-borne pathogens, such as Vibrio and Shigella Spp. NDM-1 producing isolates have already been identified that are resistant to all antibiotics except colistin and tigecycline.
Inhibitors for MBLs are not yet commercially available. MBL-mediated hydrolysis does not involve a covalent acyl-enzyme intermediate, which is in contrast with the hydrolytic mechanism of serine β-lactamases. Thus, the strategy of generating stable acyl-enzyme analogues which has provided inhibitors for the serine enzymes is not applicable to MBLs. In contrast to the serine carbapenemases, there is no MBL inhibitor in clinical development. The structural variability present among MBLs has prevented generalizations regarding substrate binding and hydrolysis, thus hampering the development of effective inhibitors.
Despite sharing a common 3D fold, MBLs exhibit significant sequence diversity. This diversity has led to the definition of 3 MBL subclasses (B1, B2 and B3) varying in the arrangement of metal ligands in the active site structure, zinc stoichiometry, loop architecture and their activity with respect to different β-lactams. Consequently, while numerous groups have evaluated a wide variety of scaffolds as potential MBL inhibitors, variation between MBLs, even in the same Subclass, results in differences in inhibitor potency of several orders of magnitude.